A two-dimensional phase-field study on dendritic growth competition under convective conditions

Abstract

In the present work, the growth competition of columnar dendrites is investigated in the presence of melt convection. By using a multiphase-field model, the microstructural evolution of unidirectionally solidified dendrites with different crystal orientations is addressed in detail. Generally, for converging bicrystal grains, the dendritic growth competition follows a conventional overgrowth model, where a favorably oriented dendrite eliminates an unfavorably oriented dendrite at the grain boundary. However, in the presence of lateral melt flow in the liquid phase, we discover that the conventional overgrowth mechanism transforms into an unusual overgrowth mechanism as the melt velocity gradually increases. Recently observed in bi-crystal studies, the unfavorably oriented dendrites overgrow at the expense of the favorably oriented dendrites during an unusual overgrowth behavior. From our simulations we elucidate that the presence of an additional convective transport in the liquid phase modifies the solute distribution at the grain boundary, which in turn affects the overgrowth mechanism of converging dendrites. We show that there exists a critical melt velocity to determine the growth competition at the grain boundary. In addition, the role of interfacial anisotropy on growth competition is analyzed in detail, where we discern that the conventional overgrowth behavior is dominant at large anisotropic strengths. Lastly, a morphological selection map is depicted to predict the crossover region between the two overgrowth behaviors.